Air would make your brakes spongy, you would notice that right away. The reasons that cause the brake line to come on:
* Emergency brake set
* Leak in system
* brake line taking up too much of the pressure (brake hose ballooning)
* A lot of air in the brake system somewhere (if it's excessive to one of the front calipers, you would feel the car pulling to one side or the other during braking, this will at least narrow down this particular symptom to the front or rear brakes.)
* Proportioning valve not working correctly.
The prop valve is what turns the brake light on (other than the emergency brake). I would not suspect the booster or master cylinder, though others with more experience may if they have encountered some weird situation that could explain it.